What is the primary function of the cell membrane?

The primary function of the cell membrane is to regulate the entry and exit of substances. The cell membrane, also known as the plasma membrane, is a selectively permeable barrier that surrounds the cell, helping maintain the internal environment by controlling which molecules can pass in and out. This selectivity is crucial for maintaining homeostasis, as it allows essential nutrients to enter the cell while keeping harmful substances and waste materials from crossing the membrane. The structure of the cell membrane, primarily composed of a phospholipid bilayer with embedded proteins, facilitates this process by allowing some molecules to pass freely while restricting others, based on factors like size and polarity.